In a digital audio apparatus such as a CD player, a D/A conversion circuit is used for performing digital-to-analog conversion on a digital signal recorded on a recording medium such as a CD to perform audio reproduction.
In general, the D/A converter does not always have ideal conversion characteristic. In most cases, a conversion error arises and the conversion involves quantization noise. Such a noise component causes distortion of a reproduced signal. To avoid this disadvantage, a method of mixing a dither signal in a digital signal is known.
FIG. 11 shows an example of a conventionally known D/A conversion circuit using a dither generator. As shown in FIG. 11, a digital signal picked up from a recording medium is fed through a signal processing LSI 1 to a digital filter 2 by which the sampling rate or frequency f.sub.S is converted to be N times as high as f.sub.S. Then the signal is converted into an analog signal by a D/A converter 4.
An adder 3 is disposed between the digital filter 2 and the D/A converter 4. Random M-series dither (noise) from a dither generator 7 is added to (or superimposed on) the digital signal by the adder 3 to attain whitening of quantization noise and suppression of a conversion error. Because the dither once added must be removed afterwards, the D/A converter 4 is followed by a subtracter 5 by which the dither supplied from the dither generator 7 is subtracted (or removed) after digital-to-analog conversion by another D/A converter 8. The analog signal thus purified is sent out through an analog low-pass filter 6. The analog low-pass filter 6 serves to cut the signal component of over-sampling frequency Nf.sub.S when the sampling frequency f.sub.S is made high by a factor N (over-sampling).
In the above-mentioned conventional D/A conversion circuit, generally, the D/A converter 4 does not operate when it does not receive a signal. Accordingly, the converter 4 does not produce noise pertaining to its switching operation. Accordingly, the converter 4 has a good S/N characteristic. However, the dither signal of a constant level is always fed from the dither generator 7 to the adder even if the converter 4 does not receive an input signal (hereinafter referred to as "non-input-signal time"). Consequently, the mixed dither signal unnecessarily forces switching operation on the D/A converter 4 so that noise pertaining to the switching operation is produced to deteriorate S/N in the non-input-signal time. To eliminate this disadvantage, a method of limiting the non-input-signal-time noise only to quantization noise is known (Japanese Patent Postexamination Publication No. 62-21420).
The known method can limit noise but cannot perfectly eliminate the influence of the dither signal because the non-input-signal-time noise still exists.
The aforementioned problem arises in an A/D conversion circuit as well as the D/A conversion circuit. In short, the problem caused by influence of the dither signal in the non-input-signal time cannot be neglected as well as the problem caused by quantization error.